The present invention relates to a multilayered balun (balance-to-unbalance) signal transformer which is used in a mobile communication apparatus, or the like.
A balun (balance-to-unbalance) transformer has a function of converting an unbalanced signal relative to the ground into balanced signals on a pair of lines or converting balanced signals into an unbalanced signal. The balun transformers of this type are used in transmitting circuits and receiving circuits of mobile communication apparatuses such as portable telephones, car telephones, etc.
FIG. 5A shows an example of use of such a balun transformer and an example of a transmitting circuit in a portable telephone. A voice modulation signal 30 and an oscillation signal of a voltage-controlled oscillator 31 are mixed with each other by a mixer 32. The output of the mixer 32 is made to pass through a band-pass filter 33. An unbalanced signal which is the output of the band-pass filter 33 is converted into balanced signals by the balun transformer 34, so that the balanced signals are supplied to a power amplifier 35 which is a balanced signal input-unbalanced signal output type. The reference numeral 36 designates a coupler for detecting the level of a transmission signal; 37, an AGC circuit for controlling the gain of the power amplifier 35 on the basis of the level of the transmission signal detected by the coupler 36; 38, a low-pass filter; and 39, a duplexer for separating the signal into a transmission signal and a receiving signal by frequencies. Such a balun transformer 34 may be provided between the mixer 32 and the band-pass filter 33. If the balun transformer 34 is used to provide balanced signals to be used in the power amplifier 35, there arises an advantage that the level of noise can be reduced compared with the case where an unbalanced signal is used.
As the aforementioned balun transformer 34, there is heretofore used a configuration as shown in FIG. 5B in which two transformers 40 and 41 are provided between a pair of balanced signal terminals 1a and 1b and an unbalanced signal terminal 2, a configuration as shown in FIG. 5C in which a transformer 42 is provided between a pair of balanced signal terminals 1a and 1b and an unbalanced signal terminal 2 and in which an intermediate tap of a coil 43 between the balanced signal terminals is connected to the ground, or the like. Although the aforementioned balun transformer has been conventionally achieved by a wire-wound type transformer, a configuration in which a balun transformer is formed from a multilayered structure containing an electrical conductor pattern formed between electrically insulating layers to make coils contained in a chip to thereby form a transmission line type transforming circuit has been described, for example, in JP-A-3-153011, etc.
The aforementioned conventional balun transformer formed from a multilayered structure is excellent both in reduction of size and in mass production. In order to achieve better characteristic in the transmission line type transformer, however, the degree of coupling between coils is necessary to be enhanced. It is very difficult that such high coupling is achieved by a multilayered structure. Better characteristic than that of the wire-wound type balun transformer has been not achieved. That is, in the case of a multilayered structure, a large loss occurs to bring a problem that phase difference or level difference cannot be suppressed when the balanced signal terminal side is used as the output terminal side. Further, when the winding ratio and inductance are to be set in the case where coils constituting the transformers 40 and 41 shown in FIG. 5B or the transformer 42 shown in FIG. 5C are contained in a multilayered structure, it is difficult to design the winding ratio, inductance value, etc.